Nuclear facilities at the end of their useful lives cannot merely be abandoned. As such, to protect the public from remaining hazardous materials the site must be decommissioned which includes decontamination, dismantling and demolition with subsequent return to green field status. Usually the first stage in nuclear plant decommissioning is the removal of fuel, followed by the initial wash out of the coolant system and then in situ decontamination for removing residual active species before dismantling the facility.
In the process of decommissioning a breeder nuclear reactor, the liquid metal coolant, which may be an alkali metal such as sodium or a sodium-potassium alloy, presents unthoughtof problems for disposal. Liquid sodium and/or sodium/potassium alloys are extremely reactive metals subject to highly exothermic reactions with water which may result in the generation of hydrogen gas. Accordingly, when liquid alkali metal coolants are involved in the decommissioning of a nuclear reactor additional precautions must be taken for disposing of the large quantities of alkali metal wastes.
In a breeder nuclear reactor a liquid metal coolant is used in several different areas but always for its cooling and/or heat transferring capabilities. The core of the reactor which contains the fuel element pins and the uranium-238 blanket surrounding the core are cooled by liquid metal coolant which circulates in two separate and distinct coolant loops, namely the primary and secondary or intermediate loop. The primary and secondary loops are isolated from each other to reduce the transfer of radioactive isotopes between the loops. The primary coolant loop surrounds the fuel core for absorbing heat from fission activity within the core and this coolant may contain radioisotopes of the liquid metal due to absorption of neutrons. The coolant enter the primary loop at about 600.degree. F. and leaves the core at about 900.degree. F. This absorbed heat retained by the molten alkali metal in the primary loop is transferred to the secondary or intermediate coolant loop by means of a heat exchanger. An estimated 75,000 gallons of liquid alkali metal coolant must be drained from the combined coolant loops and the liquid metal deactivated.
After the initial draining of the molten liquid coolant, the primary and secondary loops and any additional equipment have to be decontaminated in situ. In this regard, any scale or deposits of remaining solidified alkali metal need to be dissolved and removed from the coolant system.
Additionally, alkali metal especially sodium bonded fuel found within spent fuel elements must be deactivated. Fuel elements used in breeder reactors include uranium-235 pencil like pellets that are inserted into a thin-walled stainless steel tube. Included in these tubes is a small amount of an alkali metal, such as sodium which functions as a heat-transfer agent. The tube is welded shut and as more and more of the uranium-235 undergoes fission, fissures develop in the fuel allowing the alkali metal to enter the voids. The sodium extracts an important fission product, namely cesium-137, and hence become intensely radioactive.
The liquid alkali metal drained and removed from the coolant system and/or removed from spent fuel elements must be disposed of in a safe and secure manner. However, before final disposal, the alkali metals must be deactivated, especially sodium, to reduce its reactivity.
Several methods have been suggested for treating the sodium or sodium-potassium alloy to deactivate before disposal. U.S. Pat. No. 4,032,614 discloses a method for contacting molten alkali metal with a caustic solution thereby forming an alkali metal hydroxide. However, this method is carried out at increased temperatures with a concomitant production of hydrogen gas. The high temperatures used in this process increase the possibility of a hydrogen explosion thereby presenting an additional safety hazard. Furthermore, the method produces large quantities of caustic material which is considered hazardous due to its corrosivity. As such, disposal becomes a problem because the Environmental Protection Agency considers this caustic material as "mixed waste" due to its hazardous characteristics and radioactive content. Accordingly, the caustic material has to be disposed of in a hazardous waste site. Still further, this method has the limitation of not being applicable for dissolving and removing any deposited alkali metal remaining on process equipment, tools or in the circuit loops of the reactor.
U.S. Pat. No. 5,678,240 overcomes the problems presented when producing alkali metal hydroxides by further converting the caustic waste materials to alkali metal carbonates. This method eliminates the concern for disposal of hazardous corrosive materials but includes several steps that involve the initial conversion to a hydroxide. As such, concerns for generating explosive hydrogen gas is still applicable. Furthermore, this method may not be used for final wash out of a reactor's coolant systems to remove any remaining scale or solids.
Accordingly, a need exists for improved methods for deactivation of metal coolants removed from a nuclear reactor that reduces the production of explosive hydrogen gas and/or hazardous caustic materials, do not leave solid deposits and residue on process equipment after deactivation and may be used in a final in situ decontamination of a reactor's coolant systems.